Recently, evidence has accumulated that the hypoxic cells in solid tumors undergo reoxygenation following irradiation and become susceptible to subsequent irradiation, and that the extent of such reoxygenation of hypoxic cells may be the determining factor in the curability of tumurs by radiotherapy. Although it has often been mentioned that vascular function might play an important role in the reoxygenation process, systematic investigation of the relation between vascular function and reoxygenation in the irradiation tumors has not been done. In the present study, the sequential change in the oxygenation and in the various aspects of vascular function following X-irradiation will be investigated in various mice and rat tumors. Specifically, the functional intravascular volume and vascular permeability will be measured simultaneously by Cr51-RBC and I125-plasma protein respectively. The proportion of hypoxic cells in the tumors before and after the irradiation will be quantitated from the cell survival curves obtained by in vitro colony formation in agar media and/or by the end- point dilution method. This information should enable us to elucidate the effect of vascular change on reoxygenation and thus on tumor radioresponse, and to find a means to overcome the hypoxic protection of tumors in clinical radiotherapy.